The present invention relates to a shielded flat cable in which effective countermeasure is taken to reduce noise levels of high frequency signals.
High frequency oscillators, for instance, require miniaturization with an increase of miniaturization of electronic apparatuses or the like. To such requirements, a shielded flat cable in which signal wirings (strip lines) are disposed and insulated from each other between insulating layers has been developed. When the signal wirings are formed between the insulating layers (internally formed), there is a likelihood that generated signal radiation adversely affects other signal wirings. In addition, an influence of an external electromagnetic noise may cause a malfunction in a wiring circuit including the signal wiring.
To deal with such problems, a configuration of a shielded flat cable is proposed. In the configuration, insulating and shield layers are concentrically stacked with each signal wiring to form a shield wire. Thereafter, the shield wires are disposed in parallel and covered by an insulator to integrate, thereby forming a shielded flat cable. The signal wiring (center conductor) is made of tin plated annealed copper wire or silver plated copper alloy wire, and the shield layer is made of a mesh of tin plated annealed copper wires. Furthermore, the aforementioned shield wires are arranged in parallel with a pitch of approximately 1.27 mm, followed by covering with an insulating layer of such as polyvinyl chloride resin to integrate with a thickness of approximately 2 mm.
Though the shielded flat cable mentioned above has an ability of reducing an adverse influence due to the signal wirings, there are still the following problems. That is, even in this kind of shielded flat cable, higher densification or finer patterning of signal wirings is required to provide greater compactness or higher functionality of the circuit.
However, in making the circuit more compact or denser, more complicated machine operations, and finer and more precise machining or the like are necessitated, thereby tending to result in a remarkable increase of the manufacturing costs or decrease of reliability. In other words, in the manufacture and formation of the aforementioned shield wirings, there is a limit in dimensional accuracy in the parallel arrangement of the shield wirings and in the covering and integration with the insulating layer. As a result, the densification is largely restricted.
As a means for stabilizing the signal wirings disposed inside the insulating layer, there is proposed the following configuration. That is, in the configuration the signal wirings are arranged sandwiched in plane between ground wirings, and are sandwiched from above and below between a pair of shield layers. In addition, the ground wiring and shield layer are electrically connected by means of a vertical shield conductor (via interconnection).
One shield layer is a grounding layer made of copper foil disposed integrally on the other main surface of the insulating layer on which the signal and ground wirings are disposed. The other shield layer is a conductive paste layer or the like disposed integrally on the other main surface of the insulating layer covering the signal wirings. However, in the configuration where the signal and ground wirings are sandwiched from right to left in plane and from above and below, the following operations are prerequisite. That is, when vertically connecting the ground wiring with the shield layer by means of a wiring conductor, it is suggested to bore in advance, at a corresponding position, a necessary hole by drilling, followed by the formation of a conductor layer or the like inside the hole.
In boring by means of a drill, a small diameter of approximately several hundreds xcexcm is a lower limit to bore. This small diameter not only becomes an obstacle in higher densification and miniaturization of the signal wirings or the like, but also exerts a large adverse influence on yield or the like, resulting in an increase of manufacturing cost. A small hole of approximate 300 xcexcm can be bored by means of laser machining in place of the boring due to the drilling. However, it is difficult to form a connection of high reliability through the hole.
The present invention is carried out in view of the aforementioned situations. The object of the present invention is to provide a shielded flat cable that has a simplified structure and high shielding reliability, and a manufacturing method thereof.
A first aspect of the present invention is a shielded flat cable, comprising an insulating layer that comprises liquid crystal polymer and is folded and stacked integrally, signal wirings disposed on a folded surface of the insulating layer and ground wirings disposed thereon, the signal wirings and the ground wirings being insulated from each other, a shield layer integrally disposed on an external surface of the insulating layer to cover a disposition area of the signal wirings and ground wirings, and a conducting portion piercing through the insulating layer to connect electrically the ground wiring with the shield layer.
In the shielded flat cable of the present invention, the shield layer may be excised at a plurality of positions corresponding to a fold area of the insulating layer.
A second aspect of the present invention is a manufacturing method of a shielded flat cable, comprising the steps of forming a wiring base substrate in which signal wirings and ground wirings are disposed and insulated from each other on one side area or a center area of one main surface of an insulating layer comprising liquid crystal polymer, and a conductive foil layer connected with the ground wiring which is disposed on the other main surface of the insulating layer, folding the wiring base substrate along the outside of a disposition area of the respective wirings so that a non-disposition area of the respective wirings faces the disposition area thereof, and bonding integrally opposing surfaces of the folded wiring base substrate to make the conductive foil layer a shield layer.
A third aspect of the present invention is a manufacturing method of a shielded flat cable, comprising the steps of, forming signal wirings and ground wirings insulated from each other on one side area or a center area of one main surface of an insulating layer comprising liquid crystal polymer, aligning a conductive foil layer having a conductive protrusion capable of connecting with the ground wiring on the other main surface of the insulating layer, and stacking to form a stacked body, compressing the stacked body integrally to bring the conductive protrusion into piercing through the insulating layer and electrical contact with the ground wiring, and forming a wiring base substrate, folding the wiring base substrate along the outside of a disposition area of the wirings of the insulating layer so that a non-disposition area of the wirings faces the disposition area thereof, and bonding integrally opposing surfaces of the folded wiring base substrate to make the conductive foil layer a shield layer.
In the manufacturing method of the shielded flat cable of the present invention, an insulating adhesive layer may be interposed between the opposing surfaces in the step of bonding integrally.
A part of the conductive foil layer corresponding to a fold area of the insulating layer may be excised in the step of bonding integrally.
In the present invention, the signal wiring, ground wiring and shield layer are composed of conductive metal such as copper, aluminum or the like and is generally formed in foil or thin film of a thickness of approximately from 12 to 35 xcexcm. The signal wiring, ground wiring and shield layer are formed by patterning a copper foil of a copper clad liquid crystal polymer film for instance.
In general, widths of the signal and ground wirings are in the range of approximately 110 to 120 xcexcm and a distance (pitch) between the signal wiring and the ground wiring is also in the range of approximately 110 to 120 xcexcm. For the disposition area of the signal wiring and the ground wiring, in the case of the insulating layer being folded in two, one area (single side area) of two areas is selected. In the case of the insulating layer being folded in three, a center area of three areas is selected. However, in the above division, the folded non-disposition area need only be sufficiently large to correspond to the wiring disposition area. In this meaning, strict two or three division is not meant.
Furthermore, the shield layer for shielding an area where the signal wiring and the ground wiring are disposed is formed by folding a sheet of conductive foil layer or thin layer. That is, at least one external edge periphery in a length direction of one shield layer is extended and the extended portion is folded to form opposing shield layers, thereby a necessary shield potential being maintained.
Specifically, by setting for the width of the insulating layer to exceed two times the width of the wiring disposition area and by disposing the conductive foil layer or the like over an opposing surface to the surface on which the wirings are formed, a wiring base substrate is formed. The non-disposition area is folded with the conductive foil layer disposed outside. Thereby, the wiring disposition area is covered by and integrated with the wiring non-disposition area through the insulating layer. The opposing surfaces of the folded insulating layer may be integrated due to thermal fusion of the liquid crystal polymer forming the insulating layer. By interposing an adhesive resin layer of coating type or film type of epoxy resin or the like, the integration can be implemented more easily. The shield layer, in the case of folding in three, is preferable to be disposed over an entire circumference of the wiring disposition area. However, in the case of folding in two, the shield layer may be excised in a narrow band shape in one side of the wiring disposition area (next to the ground wiring).
When folding integrally the insulating layer and the conductive foil layer (shield layer), the excision of part of the conductive foil layer corresponding to the fold area, that is a center portion in a folding direction, improves the folding property of the shielded flat cable. The partial excision of the conductive foil layer is appropriately implemented within the range capable of maintaining electrical continuity of the conductive foil layer.
In the present invention, for the insulating layer in which the signal and ground wirings and conductors for connecting the ground wiring with the shield layer are internally disposed, liquid crystal polymer is used. That is, the liquid crystal polymer, being almost non-hygroscopic, approximately 3.0 (1 MHz) in dielectric constant and stable in a broad frequency range, is suitable for high frequency cables.
The liquid crystal polymer is multi-axially oriented thermoplastic polymer typical in for instance XYDAR (Product Name of Dartco Co.,) and VECTRA (Product Name of Clanese Co.,). Other insulating resin may be added or compounded to the liquid crystal polymer to be denatured. A film thickness thereof (thickness of insulating layer) is in the range of 30 to 100 xcexcm for instance.
The liquid crystal polymer is different in its melting point or the like due to its molecular structure. Even under the same molecular structure, due to the crystal structure or additives, the melting point varies. For instance, Bectran A (Products of KK Kurare. Melting point: 285xc2x0 C.), Bectran C (Products of KK Kurare. Melting point: 325xc2x0 C.), BIAC film (Products of Japan Goatex Co. Ltd. Melting point: 335xc2x0 C.) or the like can be cited.
In the embodiments of the present invention, since the signal and ground wirings are integrated with the shield layer, the shielded flat cable is compactly structured and simplified in the manufacturing process. Furthermore, since mechanical and electrical connections can be assuredly secured with ease, they function as the shielded flat cable for high frequency having shielding property of high reliability.
In the inventions set forth in claims 3 through 6, while simplifying the manufacturing process and saving labor therein, the shielded flat cable for high frequency with high reliability can be provided with ease and high yield.